well being married to the avatar means she's seen plenty of firebending. Not to mention she's fought several firebenders because of the war. So it's safe to say she knows a strong firebender when she see's one.

omg
just
really
thats your argument
im sorry i have a point
im sorry i miss read it
im sorry korra looks manish
i know cause she looks like someone i know
WHO LOOKS MANISH AS ****
if im "annoying" you
**** off man

They were scheduled to have the entire story arc fit within 14 episodes, they weren't scheduled to have more until the story was written. Would you rather they had decided to make an entire series with no conclusion?

This is incorrect. The fortuneteller told her that she will pass away quietly after having her third GREAT grandchild.
So far we haven't seen any sign of great grandchildren. Jinora (which at the moment is the oldest grandchild is only 10 years old at the time, and won't probably reproduce before at least 10 years) . Because of this, I wouldn't be surprised to see Katara live to be at least 100 (she is 85 in the first season of LoK).

Remember that Tenzin is the youngest of Katara's three kids.
Katara is in her eighties. Tenzin is 40-50. That means Bumi must be around 60 years old, which is definitely old enough to have grandchildren. Also we don't know anything about Katara's daughter, Kya. She could also have adult children.